Inverse Multiplexing for Asynchronous Transfer Mode (IMA), as defined in the IMA Specification Version 1.1 (AF-PHY-0086.001) published by the ATM Forum in March 1999, is one example of a multiple-link communication scheme. According to IMA, multiple physical communication links are configured into an IMA group to form a single logical or virtual link. At a transmitting end of an IMA virtual link, an ATM stream is inverse multiplexed or “split” and transmitted over the multiple links. The ATM stream is then reassembled at a receiving end from ATM cells that are received on the multiple links. IMA might be used to allow an ATM stream from a large bandwidth link to be split for transmission over lower bandwidth links such as multiple DS1 links, and subsequently reassembled for transmission on another large bandwidth link, for example. Since correct reassembly of an ATM stream requires ATM cells to be extracted from multiple links, these links must be properly synchronized.
Many communication network devices provide redundancy protection for certain components to limit the effects of failures. Switching between redundant communication ports, for example, might be handled by an Automatic Protection Switching (APS) system when a port itself fails or a communication medium through which the port communicates with a remote port degrades. Line cards and/or other components could similarly be protected through such physical redundancy and automatic switching arrangements.
When a protection switch occurs, IMA cells may be lost or duplicated. As a result of cell loss or duplication, a link in an IMA group might not maintain synchronization, which results in that link going into out of IMA frame (OIF) state, and then loss of IMA frame (LIF) state. As noted above, correct ATM stream reassembly requires all IMA links to be functioning properly, and accordingly cell loss or duplication on one link significantly affects user data traffic flowing over the IMA virtual link.
Recovery of synchronization after cell loss or duplication is not specifically addressed by the IMA Specification Version 1.1. The specification does address four different cases of problems with ICP cells, which are used for link synchronization.
As described in the IMA Specification Version 1.1, (see FIG. 19 and page 69 of the Specification), the process for IMA Frame Synchronization deals with four cases of problems with ICP cells. Those skilled in the art will be familiar with IMA frames and with ICP cells, which are used for link and group synchronization in IMA. The problem cases noted in the Specification include: ICP cell in an unexpected position in an IMA frame, ICP cell missing, ICP cell(s) invalid, and ICP cell(s) errored. Each of these conditions might result in a transition into an IMA “Hunt” state, which is one variation of link reset, to regain link synchronization. One problem with transitioning into the Hunt state, especially with a large number of links, is that it can take longer than an acceptable amount of time to achieve link synchronization. In some cases it can take several seconds to regain link synchronization.
One commercially available chipset appears to be tolerant to some cell loss, but behaves in accordance with the IMA Specification for ICP cells that are in an unexpected position in the frame, invalid, or errored. While in the IMA Sync state defined in the Specification, this chipset continually examines ICP cells for each frame. If a certain number of received consecutive ICP cells have Header Error Correction (HEC) or Cyclic Redundancy Check (CRC)-10 errors, i.e., are errored ICP cells, then the IMA Hunt state is re-entered. The Hunt state is also re-entered if a number of received consecutive ICP cells are invalid, if a non-ICP cell is received at the expected ICP cell position, or if a valid ICP cell is received at an unexpected position.
It will be apparent from the foregoing that the normal recovery mechanism used for loss of synchronization on a link of an IMA group is to reset the link. The typical reset procedure duration, on the order of a second, is not acceptable for many applications and services. Another impact of cell loss or duplication is that IMA group frame alignment is lost. This can result in data traffic being out of the correct order when it is extracted from the multiple links. Again, the usual recovery mechanism for this state is to reset the links within a group to recover alignment. Resetting all of the links in an IMA group can have an even greater impact on user traffic because the entire IMA group remains down during re-negotiation of group configuration with far end communication equipment.
Link resets may also have additional effects. For example, resetting one or all links of an IMA virtual link may cause other communication equipment that transmits traffic to or receives traffic from an IMA virtual link to reset. Network elements in a wireless communication network for which IMA virtual links are used for backhaul, for instance, might reset and thus drop calls for cellular customers in the event of an IMA link reset.
Cell loss and duplication, and the resulting loss of synchronization, may have other causes than protection switching. It should also be noted that multiple-link communication mechanisms other than IMA may be affected by synchronization loss.
Thus, there remains a need for improved synchronization recovery techniques for multiple-link communications.